Top Voted Answer

You do need the sol blade (and himi to use the search psynergy at the inside) you get the sol blade after you are at the apolo sanctum just go along with the mane story line and you will run into the weapon (only mathew can yield it)

Answers

The only way to reach Otka Island is to allow the whirlwind blowing around the Otka Sea to carry your ship to the Island. You cannot advance into the dungeon until you get the Sol Blade, very late in the game.

In the FAQs section on this site there are three maps that you can use to navigate Otka Island. You can find Mythril Silver, Power Bread, Triton's Ward, Psy Crystal and a new summon (as well as a strong boss) in the dungeon.